The need for better understanding of the low-frequency unsteadiness observed in shock wave/turbulent boundary layer interactions has been driving research in this area for several decades. This work investigates the interaction between an impinging oblique shock and a supersonic turbulent boundary layer via large-eddy simulations. Special care is taken at the inlet in order to avoid introducing artificial low-frequency modes that could affect the interaction. All simulations cover extensive integration times to allow for a spectral analysis at the low frequencies of interest. The simulations bring clear evidence of the existence of broadband and energetically-significant low-frequency oscillations in the vicinity of the reflected shock, thus confirming earlier experimental findings. Furthermore, these oscillations are found to persist even if the upstream boundary layer is deprived of long coherent structures. Starting from an exact form of the momentum integral equation and guided by data from large-eddy simulations, a stochastic ordinary differential equation for the reflectedshock foot low-frequency motions is derived. This model is applied to a wide range of input parameters. It is found that while the mean boundary-layer properties are important in controlling the interaction size, they do not contribute significantly to the dynamics. Moreover, the frequency of the most energetic fluctuations is shown to be a robust feature, in agreement with earlier experimental observations. Under some assumptions, the coupling between the shock and the boundary layer is mathematically equivalent to a first-order low-pass filter. Therefore, it is argued that the observed lowfrequency unsteadiness is not necessarily a property of the forcing, either from upstream or downstream of the shock, but simply an intrinsic property of the coupled dynamical system.

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Small spacecraft with limited on-board resources would benefit greatly from the development of a low power, low cost microthruster able to offer propellant savings over conventional alternatives and enable higher energy missions. Such a thruster would also be beneficial in the development of all-electric spacecraft whereby the normally separate reaction control system and primary electric propulsion system were able to operate from a common propellant management system. In recent years experiments on a T6 hollow cathode have demonstrated the possibility of such a device however the performance, in particular thrust efficiency, falls far short of what would be termed a viable thruster. Nevertheless the means by which hollow cathode discharges are able to produce thrust corresponding to very high velocity propellants is not at all understood, nor is the means by which to improve performance. The relevance of the very high energy ion signatures typical of hollow cathode discharges, synonymous with hollow cathode erosion, has also not been established. Indirect thrust measurements were made at the University of Southampton on two separate types of hollow cathode, derived from the T5 and T6 gridded ion thrusters, with unique design modifications, primarily of anode geometry. Testing was conducted with argon and xenon and measurements were made via a deflected pendulum micro-thrust balance and supporting architecture constructed specifically for the work. Ion energy measurements were made using a hemispherical energy analyzer in a separate experiment at NASA Jet Propulsion Laboratory on a third XIPS cathode, derived from the XIPS 25cm ion thruster, with xenon and krypton. These measurements provided unique insight into the influence of terminal parameters such as discharge current, mass flowrate and cathode/anode geometry on thrust production and downstream ion energy distributions. Significant improvements in thrust efficiency have been made with the T5 cathode and in doing so, have taken hollow cathodes a significant step forwards as a viable and competitive propulsion technology. An analysis of the thrust production is made, and on this basis conclusions are drawn on the existence of electrothermal, electromagnetic and electrostatic mechanisms specific to the cathode and operating regime, as well as their roll in other associated hollow cathode phenomenon. The main conclusions of the work are presented and recommendations made for future experimental work.

This research was motivated by the move of aero-engine manufacturers to provide services as well as products. With leasing arrangements such as TotalCare©, the aero-engine manufacturers are responsible for the operation, management and maintenance of their products while airlines pay a contracted rate for their use. As a result, aero-engine manufacturers need to minimise the cost their products incur over their lifecycle to increase profits. It is widely accepted that the greatest scope to reduce costs is in the design stage. Hence, the aim of this thesis is to create tools and methodologies for designers which will allow them to monitor the impact of design decisions on whole life cost. Two different approaches to designing for whole life cost were presented. The Life Cycle Cost (LCC) approach and the comparatively novel Value Driven Design (VDD) approach. It was observed from research literature that models for both LCC and VDD need to be tailored to specific objectives in order to keep the scope of the model manageable. This makes generic LCC or VDD models unfeasible and consequently the reuse of these models is limited. With this in mind, a methodology was developed for creating integrated analyses models which were customisable, modular and transparent so as to facilitate future modification and reuse. It used a commercial software integration package called Isight, and modular analyses modules. Model Based Systems Engineering (MBSE) was also used in the development of the integrated model architecture. Case studies were performed for both LCC and VDD approaches to allow comparison of their respective merits and flaws. Finally several avenues of future work in VDD and MBSE were discussed.

The research carried out as part of this EngD is aimed at understanding the high temperature materials used in modern gas turbine applications and providing QinetiQ with the information required to assess component performance in new propulsion systems. Performance gains are achieved through increased turbine gas temperatures which lead to hotter turbine disc rims and blades. The work has focussed on two key areas: (1) Disc Alloy Assessment of High Temperature Properties; and (2) Thermal Barrier Coating Life Assessment; which are drawn together by the overarching theme of the EngD: Lifing of Critical Components in Gas Turbine Engines. Performance of sub-solvus heat treated N18 alloy in the temperature range of 650°C to 725°C has been examined via monotonic and cyclically stabilised tensile, creep and strain controlled low cycle fatigue (LCF) tests including LCF behaviour in the presence of a stress concentration under load-control. Crack propagation studies have been undertaken on N18 and a particular super-solvus heat treatment variant of the alloy LSHR at the same temperatures, in air and vacuum with 1s and 20s dwell times. Comparisons between the results of this testing and microstructural characterisation with RR1000, UDIMET® 720 Low Interstitial (U720Li) and a large grain variant of U720Li have been carried out. In all alloys, strength is linked to a combination of γ' content and grain size as well as slow diffusing atoms in solid solution. High temperature strength improves creep performance which is also dependent on grain size and grain boundary character. Fatigue testing revealed that N18 had the most transgranular crack propagation with a good resistance to intergranular failure modes, with U720Li the most intergranular. Under vacuum conditions transgranular failure modes are evident to higher temperature and ΔK, with LSHR failing almost completely by intergranular crack propagation in air. For N18 significant cyclic softening occurs at 725°C with LCF initiation occurring at pores and oxidised particles. An apparent activation energy technique was used to provide further insights into the failure modes of these alloys, this indicating that, for N18 with 1s dwell, changes in fatigue crack growth rates were attributed to static properties and for LSHR, with 20s dwell in air, that changes were attributed to the detrimental synergistic combination of creep and oxidation at 725°C. Microchemistry at grain boundaries, especially M23C6 carbides, plays an important role in these alloys. Failure mechanisms within a thermal barrier coating (TBC) system consisting of a CMSX4 substrate, PtAl bond coat, thermally grown oxide (TGO) layer and a top coat applied using electron beam physical vapour deposition have been considered. TGO growth has been quantified under isothermal, two stage temperature and thermal cyclic exposures. An Arrhenius relation was used to describe the TGO growth and produce an isothermal TGO growth model. The output from this was used in the QinetiQ TBC Lifing Model. Thermo-mechanical fatigue test methods were also developed including a novel thermocouple placement permitting substrate temperature to be monitored without disturbing the top coat such that the QinetiQ TBC Lifing Model could be validated. The importance of material, system specific knowledge and performance data with respect to a particular design space for critical components in gas turbine engines has been highlighted. Data and knowledge regarding N18, LSHR and TBC systems has been added to the QinetiQ’s databank enhancing their capability for providing independent advice regarding high temperature materials particularly in new gas turbine engines.

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This research contributes to the knowledge on aerodynamic wing - wheel interaction. Hereto an experimental and computational study has been performed, during which the wing ride height and the wing - wheel overlap and gap have been considered as the primary variables. The wheel drag for the combined configuration is generally lower at low ride heights and higher at high ride heights compared to the case without wing. This results primarily from changes in the flow separation over the top of the wheel - partly induced by the wing circulation - from the channel flow along the inside of the wheel and from the vortex interaction in the wheel wake. The wing downforce increases at low ride heights due to the wheel presence, but reduces at high ride heights. The modified channeling effect, vortex and separation effects govern the wing flow field, although the wheel circulation acts as an additional mechanism for downforce enhancement and limitation. The wing - wheel interaction has been studied extensively for a baseline configuration, using forces, on-surfaces pressures for the wing and wheel, oil flow and PIV data. A reduced set of data has been obtained for alternative overlap and gap settings. An increase in overlap generally leads to a reduction in wheel drag and wing downforce. A larger gap setting has relatively little influence on the wheel drag at low ride heights, but shifts the higher ride height part of the curve to lower values. The wing downforce is generally slightly lower when the gap increases. An analogy between the wing - wheel configuration and a multi-element airfoil has been used to partly explain the aerodynamic interaction between the components, based on the cross flow along the flap trailing edge. The application of a steady RANS computational approach with Spalart Allmaras turbulence model has been assessed for a baseline configuration over a range of ride heights. Qualitatively, the flow field is predicted fairly accurately, but the flow quantities correlate less satisfactory with the experiments. The downstream interaction in underpredicted, resulting in lower values for the wheel drag, in particular at high ride heights. The use of non-conformal zones around the wing is one of the causes for this discrepancy.

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